batman-adv: frag: ensure fragment is writable before modifying TTL

batman-adv: frag: ensure fragment is writable before modifying TTL

Before batman-adv is allowed to write to an skb, it either has to have its
own copy of the skb or use skb_cow() to ensure that the data part is not
shared. But batadv_frag_skb_fwd() modifies the TTL even when it is shared.

Adding a skb_cow() right before this operation avoids this and can at the
same time prepare it for the modifications required to forward the
fragment.

Cc: stable@kernel.org
Fixes: 610bfc6bc99b ("batman-adv: Receive fragmented packets and merge")
Signed-off-by: Sven Eckelmann <sven@narfation.org>